The present invention generally relates to wireless communications networks, and more particularly relates to techniques for selecting channel-state information resources for use by mobile terminals in providing received-power feedback to the wireless network.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The best improvements in performance are available if both the transmitter and the receiver are equipped with and use multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
Standards for the Long-Term Evolution (LTE) wireless network have been developed by members of the 3rd-Generation Partnership Project (3GPP). The LTE standard continues to evolve to provide enhanced MIMO support. One core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. For instance, support for a spatial multiplexing mode, which can include the use of channel-dependent precoding, is introduced in Release 10 of the 3GPP standards, which includes a number of new features that are part of an upgrade to LTE technology commonly referred to as LTE-Advanced.
LTE-Advanced's spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 1.
As seen in FIG. 1, information-carrying symbol vector s, which includes symbols from each of several layers, is multiplied by an NT×r precoder matrix NTT×r. With a properly selected matrix, this precoding operation serves to distribute the transmit energy in a subspace of the NT-dimensional vector space, where the NT dimensions of the vector space correspond to NT antenna ports at the transmitter. The precoder matrix is typically selected from a codebook of possible precoder matrices, and is typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. Each of the r symbols in the symbol vector corresponds to a layer—r is referred to as the transmission rank. In this manner, spatial multiplexing is achieved, since multiple symbols can be transmitted simultaneously, using the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete-Fourier-Transform (DFT)—precoded OFDM in the uplink. Hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) can be modeled by:Yn=HnWNT×rsn+en  (1)where en is a noise/interference vector obtained as realizations of a random process. The precoder matrix WNT×r, can represent a wideband precoder, which is constant over frequency, or can be frequency selective.
In order for the mobile terminal (referred to as User Equipment, or UE, in 3GPP terminology) to generate feedback regarding the current channel conditions, a set of pre-defined channel-state-information reference signals (CSI-RS) are transmitted by the base station (referred to as an evolved Node B, or ENB, in 3GPP terminology), for measurement by the mobile terminal. Using the CSI-RS, a UE can estimate the characteristics of the propagation channel between the eNodeB and the UE and consequently also figure out which precoder best suits the particular channel.
In LTE Release 10 and later, there is support for a transmission mode for up to S-layer spatial multiplexing for eight transmission antennas, meaning that the CSI-RS will be an vector x8×1. For the purpose of CSI feedback determination, the UE assumes that each of the rows in x8×1 corresponds to an antenna port (ports 15-22) on which a CSI-RS is transmitted. The first row represents antenna port 15, second row antenna port 16 and so on. Each CSI-RS port is typically transmitted from a physical antenna of its own, meaning that there is a direct correspondence between an antenna port and a physical antenna.
To meet the increasing demand for data capacity in the networks, heterogeneous network deployments, sometimes referred to as “HetNet,” are seen as an important additional means to provide increased network capacity. With the HetNet approach, relatively low-power transmission nodes/points (often referred to as “pico” or “femto” nodes) are deployed within the coverage area of conventional macro cell nodes. This overlapping of signal coverage, which can be concentrated in user “hot spots” or in areas where signal strength from the macro network is relatively weak, permits significant extensions of high-throughput coverage within the macro network.
FIG. 2 depicts an example of macro and pico cell deployment in a heterogeneous network 100 comprising a macro cell 110 and three pico cells 120. The most basic means to operate a heterogeneous network is to apply frequency separation between the different layers, i.e. between the macro cell 110 and the pico cells 120 in the heterogeneous network 100 in FIG. 2. The frequency separation between the different layers is obtained by allowing the different layers to operate on different non-overlapping carrier frequencies. Another way to operate a heterogeneous network is to share radio resources on same carrier frequencies by coordinating transmissions across macro and pico cells. For example, certain radio resources may be allocated for the macro cells during some time period, while the remaining resources can be accessed by the pico cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the above mentioned split of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between nodes in the heterogeneous network. In LTE, an X2 interface has been specified in order to exchange different types of information between base station nodes.
The members of 3GPP are currently developing the Release 11 specifications for LTE. These developing standards will include specifications for improved support for coordinated multipoint (CoMP) transmission/reception, where transmissions and receptions are coordinated among several transmission nodes to increase received signal quality and reduce interference. CoMP transmission and reception refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites is coordinated in order to improve system performance. In some cases, the coordination is distributed, in which case equipment at the various transmission sites exchange coordination information. In other cases, the coordination is handled by a central coordinating node that sends coordinating instructions to each of the participating transmission points, as necessary.
CoMP is being introduced in LTE to improve the coverage of high data rates, to improve cell-edge throughput, and to increase system throughput. In particular, the goal is to distribute the user perceived performance more evenly in the network, by taking better control of the interference. CoMP operation targets many different deployment scenarios, supporting coordination between sites and sectors in cellular macro deployments, for example, as well as several configurations of heterogeneous deployments. For instance, with CoMP, a macro node can coordinate its transmission with pico nodes operating within the macro coverage area.
There are several different CoMP transmission schemes that are being considered. One approach is called Dynamic Point Blanking, where multiple transmission points coordinate transmissions so that a neighboring transmission point mutes transmissions on the TFREs that are allocated to UEs that are experiencing significant interference. Another approach is called Dynamic Point Selection, where the data transmission to a UE may switch dynamically (in time and frequency) between different transmission points so that the transmission points are fully utilized. In another approach, called Coordinated Beamforming, transmission points coordinate transmissions in the spatial domain by beaming the transmission power in such a way that the interference to UEs served by neighboring transmission points is suppressed. With another approach, called Joint Transmission, a given transmission to a UE is simultaneously transmitted from multiple transmission points, using the same time/frequency resource.
One common denominator for the various CoMP transmission schemes is that the network needs CSI information not only for the serving transmission point, but also for the radio channels linking neighboring transmission points to a mobile terminal. For that reason, the notion of a CoMP Measurement Set has been introduced in LTE. The CoMP Measurement Set enables the eNodeB to configure a set of CSI-RS resources that the UE will use to perform channel measurements for providing CSI feedback to the network. A CSI-RS resource, which generally corresponds to a particular transmission point, can loosely be described as a pattern of time/frequency resource elements on which a particular CSI-RS configuration is transmitted. A CSI-RS resource is determined by a combination of LTE parameters, including “resourceConfig” and “subframeConfig”, which are configured by Radio Resource Control (RRC) signaling. The CSI feedback provided by the UE can include any of several metrics, such as Channel Quality Indicator (CQI), Precoder Matrix Indicator (PMI), and Rank Indicator (RI).
It should be noted that the CoMP Measurement Set must be limited to a small number of CSI-RS resources, since the UE processing becomes increasingly complex (prohibitive) with large configurations. Accordingly, the CoMP Measurement Set will likely be limited to two or three CSI-RS resources, although larger set sizes are possible.
To obtain the full benefits of CoMP, it is important that the network (e.g., the eNodeB) is able to configure the most appropriate CoMP Measurement Set for each UE, so that the UE performs channel measurements for the correct neighboring transmission points, using the CSI-RS resources that correspond to those transmission points. Since the size of the CoMP Measurement Set will be constrained to a small number, the room for error in the configuration is small. Accordingly, it is an agreed working assumption for LTE Release 11 to introduce a new, simpler measurement and corresponding measurement report, for measurements performed on a configured set of CSI-RS resources. 3GPP has recently named these resources the “CoMP Resource Management Set.” Basic signal received power measurements (and/or received quality measurements) are thus made on a more extensive set of CSI-RS resources than could practically be assigned to a CoMP Measurement Set; by evaluating these CSI-RS based received powers/qualities for a larger set of CSI-RS resources, an eNodeB can configure the UE with the most appropriate CoMP Measurement Set. For example, the eNodeB might define the CoMP Measurement Set to include the N (e.g., two) CSI-RS resources with the highest reported reference signal received powers/qualities in the CoMP Resource Management Set. The UE can then begin the more excessive measurements and reporting to provide CSI (PMI/CQI/RI) feedback for the relatively few CSI-RS resources of the configured CoMP Measurement Set. Moreover, such reported reference signal received powers/qualities can be used to configure appropriate interference measurements in a UE; for example, it can be used to configure one, or multiple, Interference Measurement Resources (IMR), for a UE.
Accordingly, it is an agreed working assumption for LTE Release 11 that the network can configure a UE to report reference signal received powers based on measurements performed on CSI-RS resources configured in a CoMP Resource Management Set. Such a measurement could be done coherently, in which case the UE needs to know the particular CSI-RS sequence that is transmitted on the CSI-RS resource, or incoherently, in which case the transmitted actual sequence can be unknown, i.e., “transparent,” to the UE. In either case, an estimated signal power/quality corresponding to the CSI-RS resource and derived from the measurement performed on the CSI-RS resource is fed back from the UE to the eNodeB. In the discussion that follows, the fed back reference signal received power/quality values are collectively referred to as “CSI-RS received power” (CSI-RSRP), but it should be understood that CSI-RSRP encompasses any quantity that represents a received quality of a CSI-RS signal.
As mentioned above, the network configures the terminal to measure CSI-RSRP on a set of CSI-RS resources, which set is referred to herein as the CoMP Resource Management Set. In other contexts this set might also be referred to as the “RRM Measurement Set”, “CoMP RRM Measurement Set”, “CSI-RSRP Measurement Set”, or “Extended CoMP Measurement Set,” for example. The CoMP Resource Management Set provides a useful tool for the eNodeB to acquire information on which transmission points (i.e., which CSI-RS resources) are most suitable for inclusion in the CoMP Measurement Set, such as the two or three transmission points having the highest CSI-RSRP.
Even though measurements of CSI-RSRP are substantially less computationally complex than a full CSI (CQI/PMI/RI) report, there is nonetheless a UE complexity involved. For that reason, the CoMP Resource Management Set will be limited in the maximum allowed size, i.e., limited in the number of CSI-RSRP measurements the UE shall be capable of performing. A likely limit in CSI-RSRP size is on the order of five to eight.